In the prior art, as the developer of electrostatic images, there has been used the so-called two-component system developer comprising a mixture of a carrier and a toner. This developer charges the carrier and the toner through friction by stirring to polarities opposite to each other, and the charged toner is adhered electrostatically to the electrostatic image having the opposite polarity thereby to effect development, and therefore only the toner is consumed as the development is repeated, whereby the ratio of carrier to toner becomes changed. For this reason, for obtaining adequate images, the toner is required to be successively supplemented to maintain the mixing ratio at a constant value, whereby a complicated and expensive toner supplementing device must be provided. Also, as to the carrier, it is deteriorated through adhesion of fine powders of toner on its surface, etc. during usage for a long term, whereby unfavorable phenomena are brought about such as lowering of image density and increase of ground contamination. For this reason the carrier must also be exchanged periodically with new. The above problems are not substantially overcome in a developer using the so-called coated carrier having the surfaces of carrier particles coated with a resin.
In order to remove the drawback of the two-component system developer as described above, there is proposed the so-called one-component system developer containing no carrier. This developer comprises a magnetic toner, comprising fine magnetic material powders dispersed in a thermoplastic resin, optionally with incorporation of carbon black and/or a dye dispersed therein, followed by pulverization.
The magnetic toners may be broadly classified into the so-called electroconductive toner having relatively lower resistance and the so-called insulating toner having relatively higher resistance.
The important factors influencing the image quality when forming images according to xerographic process are the developing characteristics and transfer characteristics. In the aforesaid electroconductive toner, since the development proceeds based on the electrostatic induction by the electrostatic charges, no true charge is required for the toner. Therefore, there is the advantage that no harmful fluctuation of development occurs through constant fluctuations of its value depending on humidity, but there is the drawback that "blur" is formed on the transferred image by occurrence of disturbance of the electrical force line when tranferring the toner image onto a transfer paper by an electrostatic transfer means. On the other hand, the insulating toner has a true charge of the opposite polarity to the latent image charge, and development proceeds through electrical attraction between the true charge of the toner and the latent image charge. Such an insulating toner has the drawback that development characteristic is poor because of constant fluctuation of the true charge of the toner depending on humidity, but it has the advantage that a transferred image without "blur" can be obtained without disturbance of electrical force line when transferring the toner image onto transfer paper by an electrostatic means. Such electroconductive toner and insulating toner have developing characteristics and transfer characteristics which are opposite to each other, and therefore there is involved the problem that is is difficult to obtain stable images of good quality by development with the use of either one of the toners.
As a means for solving such a problem, there has been attempted, for example, the method in which an electroconductive magnetic toner is used for development, on one hand, and a coated paper with resin coating is used to prevent disturbance of the electrical force line during transfer for improvement of transfer characteristic. However, such a coated paper requires additional working resulting increased cost, and the original merit of the transfer system capable of using a plain paper is lost. Alternatively, attempts to improve developing characteristic by the use of an insulating toner have also been made.
For example, Japanese Provisional Patent Publication No.
31136/1978 discloses a developing system in which the toner is charged and the bias voltage is also applied. Also, Japanese Provisional Patent Publications No. 118056/1978 and No. 22835/1979 disclose development with a mixture of two kinds of magnetic toners with different resistances. And, Japanese Provisional Patent Publication No. 42141/1979 discloses a technique, in which developing characteristic is improved by making the toner layer at the developing section extremely thin, thereby making shorter the distance between the photosensitive member and the toner carrier (e.g. non-magnetic sleeve).
Even by the use of an insulating magnetic toner, considerable improvements of developing characteristic can be seen in the methods as mentioned above, but it has not yet reached the region of an electroconductive toner. Moreover, there also remained the drawbacks that the development device was required to be specially elaborated and that the mixing ratio of the two kinds of magnetic toners with different electric resistances was changed with the number of copies.
The present inventors have already proposed an image forming method disclosed in Japanese Provisional Patent Publication No. 168674/1981 as the method for solving these problems. That is, this method comprises performing pressing transfer of a toner image comprising an electroconductive toner formed on an image supporting member onto an intermediate transfer member, and fixing thermally the toner image by pressing transfer onto the final transfer member. According to such a method, developing characteristic is excellent due to the use of an electroconductive toner for development, and also no destruction or deformation of the three-dimensional accumulated body of the toner image is accompanied due to employment of the pressing transfer method in place of the transfer method by electrostatic means, whereby transferred images can be formed faithfully to the original images, to result in both excellent developing characteristic and excellent transfer characteristic, to find that final image of good quality can be obtained. However, when images are formed continuously, the thermal transfer conditions will be changed constantly depending on the changed in temperature or humidity conditions of the atmosphere, changes of the heating temperature of the image forming device, to find that images of good quality can not necssarily be obtained. For example, the viscoelastic property of the toner to be transferred and fixed is changed due to the temperature change during thermal transfer, whereby the softened toner cannot be sufficiently transferred onto the final transfer member (usually, transfer paper) but remains on the intermediate transfer member, which residual image is retransferred to the photosensitive member or a transfer paper subsequently delivered, etc. to cause the so-called off-set phenomenon, thereby deteriorating the image quality. Other disadvantages that occured were that fixing of the toner image was insufficient during thermal transfer, or the three-dimensional accumulated body of the toner image may be destroyed or deformed due to excessive fixing to worsen the image quality.
To cope with the unexpected phenomena as mentioned above, the present inventors have made extensive studies about the influences of thermal characteristics of the magnetic toner upon the image quality, and surprisingly found that the difference between the softening point and the softening initiating temperature of the magnetic toner is related to the image quality, and the present invention has been accomplished on the basis of this knowledge.